Add a note on custom validation contexts.

The documentation on `:on` for validations was inconsistent, and most
only referenced the `:create` and `:update` contexts. I fixed those to
be consistent with the documentation on `AM::Validations.validates`,
which seemed to have the best docs.

[ci skip]
[ci skip] add guide for ActiveModel::SecurePassword
'Reversible' ->active_record_migrations [ci skip]

Reversible method documentation moved from migrations.md  to active_record_migrations.md
'Revert' ->active_record_migrations [ci skip]

Revert method documentation moved from migrations.md  to active_record_migrations.md
[ci skip] Fix some typos, normalize sentence.

Follow up of #16722.

https://github.com/rails/rails/pull/16722

also normalize all instance gained methods’ sentence.
form_helpers: demonstrating there is also a collection_select method in form builder
[ci skip] Fix code snippet display in Action Mailer Basics guide.
Consistently capitalize ETag.
Consistently markup etag options.
Cleaner queuing of jobs using Delayed Job
Update error message for validate method

- Improve the error message by suggesting that the user may have
  intended to call validates instead of validate method.
Merge pull request #16851 from prathamesh-sonpatki/mention-validate-in-upgrade-guide

Update error message of Active Model's validate method
Move the array to a constant
Revert "Merge pull request #16888 from jejacks0n/render_template"

This reverts commit 07635a74b5ee08dcba3f6617def6230d8f114fe5, reversing
changes made to 1b5f61a025b6ce1ee52b7148e3ed2a9acbde28b9.

Reason: it's not ready :bomb:, see https://github.com/rails/rails/pull/16888#issuecomment-56400816
fix fixtures not loading if a belongs_to association is defined with a :foreign_key option that's a symbol
Better regression test for Fixtures with fk as a symbol
Merge pull request #17009 from arthurnn/foreign_key_symbols

Fixtures using Foreign key symbols
Merge pull request #16963 from collectiveidea/activejob-dj

Cleaner queuing of jobs using Delayed Job

Conflicts:
	activejob/lib/active_job/queue_adapters/delayed_job_adapter.rb
Clarify the documentation of the primary_key option for associations

Previously the documentation stated that `primary_key` should be the name of
a *method* that returns the primary key used for the association. This is
incorrect. This changes the documentation to say that the value must be the
name of a column.
Merge pull request #17012 from petedmarsh/clarify-association-primary-key-option-documentation

Clarify the documentation of the primary_key option for associations